In DARK•HERITAGE, the King in Yellow is not Hastur. There is no Hastur (that I know of yet, anyway) in DARK•HERITAGE. The King in Yellow is a title given to the being known popularly in the Terrasan language as Caronte, from ancient Terrasan Charun or Charon. His features are always obscured; he usually appears as a very tall, gaunt vaguely humanoid figure wrapped in tattered yellow rags. His face is always obscured, either by the yellow hood, or sometimes by a pallid mask. Ancient, whispered tales tell that to glimpse Caronte's true face is to go insane or drop stone dead instantly.
He's sometimes depicted riding a pale, or even sickly or skeletal horse, and carrying a staff or scythe with the yellow sign inscribed on the handle. He is commonly associated with Death.